The purpose of these studies is to determine cell volume rapidly and sequentially in toad urinary bladder and other high resistance sodium transporting epithelia. The tissue will be removed from the animal and placed in a special flow chamber in which the thickness of the mucosal chamber is 25 microns and the serosal chamber 75 microns; such narrow fluid paths allow excellent optics for microscopic observation. The tissue will then be observed using an interference contrast microscope, and the focusing knob of this device will be driven by a stepping motor. With the use of a computer, the stepping motor can be pulsed and hence the focus advanced at predetermined intervals of time and space; at each point an image will be obtained with a television camera interfaced with the microscope, and the image will be stored on video disc. With rapid pulses, an entire cell may thus be "optically sectioned" in less than 1 second, and each section stored on video disc. After completion of such a study, the conditions of the experiments can be changed and another set of images obtained on the same cell. This may be done many times. At the close of an experiment, each image will be retrieved from disc and displayed on a television monitor. The cell borders will then be traced out with a cursor interfaced with the video image and the area of a given cell can thus be determined. Knowing the distance between images, the total volume of the cell can be determined. We plan to determine cell volume as a function of sodium transport under a variety of conditions in order to determine the factors which control both.